Unstable idling when engine is hot - Amal 930 MK1 Premier

Hi,
Please, I need help from the Community.

Norton 750 Commando (1971) with Amal 930 MK1 Premier (#19, 106, 210) and Pazon Sure Fire runs great, fine throttle respons, starting - all is fine.
All but not the idling when the engine ist "hot". Sometimes the bike idles (~1200rpm), sometimes the bike stalls slowly / sometimes suddenly.

When I get some Grease on the Throttle Slides (reduced Gap) it seems to be better for a short time. So I think the Problem is in this Area.
The Carbs are 3000miles old.

My Question is: Does Anyone have the correct Diameter of the Throttle Slides and knows the Gap between the Slides and the Bore in the Carb ?
I asked at Burlen (Amal) 1 week ago without any answer ....

Thank you !

I don't have the answer to your question, but I doubt that it's worn slides in a 3000 mile Premier carb set. More likely one of:

1) no heat insulators (head to manifolds)
2) pilot circuit is partly clogged
3) air screw out to far
4) manifold crossover tube cracked (air leak) or clogged (no idle assist)
5) float (fuel) height way too low

And you're compensating by turning up the throttle stops. In other words, starving for fuel. Yes, sealing up the slide would affect this as some air gets around them even when new.

I would have put ‘out of synch slides’ at the top of the list…

Fast Eddie said:

I would have put ‘out of synch slides’ at the top of the list…

Click to expand...

Absolutely! Tricked by running great otherwise.

Tokay said:

My Question is: Does Anyone have the correct Diameter of the Throttle Slides and knows the Gap between the Slides and the Bore in the Carb ?

Click to expand...

"A note about slides:
The is a lot of folklore about the slide clearance in Amal carburetors. To insure the slide will not stick in the bore from the mounting hardware being over tightened the slides are given .0035″ to .004″ clearance. The typical comment is that Amal couldn’t hold a tighter tolerance. This is far from the truth. The early versions of the Amal Concentric had very close clearance and they experienced slides sticking when customers over tightening the mounting hardware. The .0035″ – .004″ clearance is intentional and the low throttle open jetting was compensated for the extra clearance."

marshg246 said:

I don't have the answer to your question, but I doubt that it's worn slides in a 3000 mile Premier carb set. More likely one of:

1) no heat insulators (head to manifolds)
2) pilot circuit is partly clogged
3) air screw out to far
4) manifold crossover tube cracked (air leak) or clogged (no idle assist)
5) float (fuel) height way too low

And you're compensating by turning up the throttle stops. In other words, starving for fuel. Yes, sealing up the slide would affect this as some air gets around them even when new.

Click to expand...

Thanks for the Informations

1) Insulators are installed
2) Circuit cleaned as installing the #19 idle jets, Engine reacts to changes in idle air screw
3) 1 5/8 out
4) Tested with Brake Cleaner - No leak, Idle assist ok (not clogged, used for synchronizing)
5) Float height is set (with hose), fuel level 0,22" below float chamber flange

Clip Position: Middle
Carbs synchronised with Carbtune
Valve Clearance adjusted (IN 0,10mm / OUT 0,15mm)
Ignition timing checked with strobe - ok



Bikes idles after a short time when started (Bike usually starts on the first Kick, idles slightly increased when Engine is warm ( ~5miles), idles at 1200 after >10 Miles or is sometimes unstable.
If the Engine stalls when hot and I set the throttle stops for higher revs so that the bike idles good, then it can be that the Idle is to high at the next stop.
So I set it back and at the next Stop, idling can be fine, next stop bike stalls again (slowly or suddenly).

No "plopping" or something like that in the exhaust when bike runs with closed throttles (typically when idle circuit is clogged)

Starting and idling got better with the #19 idle jets, but sometimes...

Tokay said:

Thanks for the Informations

1) Insulators are installed
2) Circuit cleaned as installing the #19 idle jets, Engine reacts to changes in idle air screw
3) 1 5/8 out
4) Tested with Brake Cleaner - No leak, Idle assist ok (not clogged, used for synchronizing)
5) Float height is set (with hose), fuel level 0,22" below float chamber flange

Clip Position: Middle
Carbs synchronised with Carbtune
Valve Clearance adjusted (IN 0,10mm / OUT 0,15mm)
Ignition timing checked with strobe - ok



Bikes idles after a short time when started (Bike usually starts on the first Kick, idles slightly increased when Engine is warm ( ~5miles), idles at 1200 after >10 Miles or is sometimes unstable.
If the Engine stalls when hot and I set the throttle stops for higher revs so that the bike idles good, then it can be that the Idle is to high at the next stop.
So I set it back and at the next Stop, idling can be fine, next stop bike stalls again (slowly or suddenly).

No "plopping" or something like that in the exhaust when bike runs with closed throttles (typically when idle circuit is clogged)

Starting and idling got better with the #19 idle jets, but sometimes...

Click to expand...

1-5/8 out is a little odd. They always are between 1-1/4 and 1-1/2 for me - usually closer to 1-1/4.

I know this is non-standard, but it works 100% the time for me on every bike I build or repair - I sync manually only:

1) Unscrew the throttle stops enough that they cannot touch the slides. Then while holding a finger on a slide turn the throttle stop in until it just touches. Then turn in 1 full turn.
2) Adjust top adjuster until slides reach the top at exactly the same time (look in the carb and watch the slide edge reach the top).
3) Adjust one throttle stop until the slides start moving off the throttle stops at exactly the same time. This normally takes only a tiny tweak.
4) Unscrew the carb adjusters until there is only a little play.
5) Go back to 2 until no adjustments are made in 2-4

If done carefully, the carbs are 100% mechanically synced and the cables are properly adjusted. Adjust the air screws to 1-1/2 out. Start the bike and once warmed set the idle to around 1100 making sure to turn the throttle stops exactly the same amount. Blip the throttle. If it stumbles, turn the air screws in slightly. Keep doing this until it's doesn't stumble, or you are less than 1-1/4 out. If you are less than 1-1/4 out, you probably have some other problem.

Hello Greg,
THANK YOU ! for the tipps and the guide line.

I will test at the next opportunity

Tokay said:

Hello Greg,
THANK YOU ! for the tipps and the guide line.

I will test at the next opportunity

Click to expand...

Hello Greg,
now I`ve tested your workaround for 2 times, looks better .... not perfect but better

Thank you !

When it's springtime here, I'll try again ....

Are you running transparent fuel lines or in line filters? If so, have a close watch while hot idling and blipping throttle. I have see through filters and can see tiny air bubbles passing up from carbs when hot. These seem to accumulate top half of filter and sometimes I see this when idle stumbles at stop lights etc. I think it has to do with heat build up in carb bodies and float bowls, even with heat insulators on head to manifold flange. Fuel is cooking/vapourizing in bowl creating an air "block" in the filter/lines. Try running with both petcocks open to see if greater fuel feed to carb bowls helps prevent effect of bubble build up in just one side.

Putting a second insulator on carb to manifolds may help reduce heat build up. Be sure you are using the thicker version on insulators (I have both thin and thick in my spares kit).

Wonder if a single carb helps with overheating, with twice the flow through the bowl?

If all carb issues are ruled out, could the problem be a coil breaking down?

Mart UK said:

Wonder if a single carb helps with overheating, with twice the flow through the bowl?

If all carb issues are ruled out, could the problem be a coil breaking down?

Click to expand...

Is it single carb in this case? When I ran single, the big hunk of alu for the 1-to-2 manifold likely contributed to carb bowl that was too hot to touch at a fuel stop. This was discovered b/c just prior to kick starting, I gave the throttle grip a twist out of habit and it did not return to closed position. The slide had become jammed in the carb bore...then I found bowl very hot. With about 2 minutes it released as things cooled down. Heat soak at fuel stop, big, low surface area of mani compared to two single mani's, and only thin insulators. I was fine again while running along road. Later placed insulators both sides of mani and never had the issue again (bowl stayed much cooler at fuel stops etc.).

Been running dual carbs for past 1.5 yrs. Still get fairly warm bowls (though never like that previous time) and I see the wee bubbles after idling for more than a minute or so at lights etc.

Tokay said:

Hi,
Please, I need help from the Community.

Norton 750 Commando (1971) with Amal 930 MK1 Premier (#19, 106, 210) and Pazon Sure Fire runs great, fine throttle respons, starting - all is fine.
All but not the idling when the engine ist "hot". Sometimes the bike idles (~1200rpm), sometimes the bike stalls slowly / sometimes suddenly.

When I get some Grease on the Throttle Slides (reduced Gap) it seems to be better for a short time. So I think the Problem is in this Area.
The Carbs are 3000miles old.

My Question is: Does Anyone have the correct Diameter of the Throttle Slides and knows the Gap between the Slides and the Bore in the Carb ?
I asked at Burlen (Amal) 1 week ago without any answer ....

Thank you !

Click to expand...

I've had sticking slides on badly worn non-premier Amal mk 1s, rectified by removing the slides and using emery paper lightly on high spots, which were visible. Haven't had this problem with premiers with anodised slides.

Is the throttle cable sticking?

Carbs not distorted through over tight manifold bolts?

@ Tornado
I`use transparent fuel lines, no Filter, 2 Carbs. Float bowles get Temperature up to 48 degrees Celsius on the outer Surface (for Example at Stop & Go or long idling)
This Temperature is "normal", I think.

@ Saan`s Aardvark
All Throttle Cables and the junction box are new, Cable and Slides are not sticking, no over tighting.

@Mart UK
Thats what I also thought, so Bike had get a new dual coil but with no change

Tornado said:

Is it single carb in this case? When I ran single, the big hunk of alu for the 1-to-2 manifold likely contributed to carb bowl that was too hot to touch at a fuel stop. This was discovered b/c just prior to kick starting, I gave the throttle grip a twist out of habit and it did not return to closed position. The slide had become jammed in the carb bore...then I found bowl very hot. With about 2 minutes it released as things cooled down. Heat soak at fuel stop, big, low surface area of mani compared to two single mani's, and only thin insulators. I was fine again while running along road. Later placed insulators both sides of mani and never had the issue again (bowl stayed much cooler at fuel stops etc.).

Been running dual carbs for past 1.5 yrs. Still get fairly warm bowls (though never like that previous time) and I see the wee bubbles after idling for more than a minute or so at lights etc.

Click to expand...

Yes, can see a larger contact area could cause that. I was thinking of my set up, which has a 'rubber' connector with jubilee clips between the carb and manifold.

I've experienced the same behavior. Wonder if the ei has something to do with it. Most work by retarding the ignition as rpm's drop. Could it be a downward spiral, lower rpm, less advance leads to lower rpm on so on til the bike dies. I've always used Pazon Surefires but have installed a Trispark on my latest bike because it is supposed to have "idle stabilization". We shall see.

Tokay said:

Thanks for the Informations

1) Insulators are installed
2) Circuit cleaned as installing the #19 idle jets, Engine reacts to changes in idle air screw
3) 1 5/8 out
4) Tested with Brake Cleaner - No leak, Idle assist ok (not clogged, used for synchronizing)
5) Float height is set (with hose), fuel level 0,22" below float chamber flange

Clip Position: Middle
Carbs synchronised with Carbtune
Valve Clearance adjusted (IN 0,10mm / OUT 0,15mm)
Ignition timing checked with strobe - ok



Bikes idles after a short time when started (Bike usually starts on the first Kick, idles slightly increased when Engine is warm ( ~5miles), idles at 1200 after >10 Miles or is sometimes unstable.
If the Engine stalls when hot and I set the throttle stops for higher revs so that the bike idles good, then it can be that the Idle is to high at the next stop.
So I set it back and at the next Stop, idling can be fine, next stop bike stalls again (slowly or suddenly).

No "plopping" or something like that in the exhaust when bike runs with closed throttles (typically when idle circuit is clogged)

Starting and idling got better with the #19 idle jets, but sometimes...

Click to expand...

valve clearances (0,10 / 0,15mm) might be a bit too tight. 0,10mm (4 thou), 0,15mm (6 thou)

The valve clearance is perhaps a little too tight, for my part, 15 and 20 are the right values to be quiet.
However, I am surprised that no one has mentioned the float bowl level, because it is a very important point to check, even on a new carburettor.

Persohn said:

The valve clearance is perhaps a little too tight, for my part, 15 and 20 are the right values to be quiet.
However, I am surprised that no one has mentioned the float bowl level, because it is a very important point to check, even on a new carburettor.

Click to expand...

"5) Float height is set (with hose), fuel level 0,22" below float chamber flange"

Could your trouble be in the stock throttle cable set-up ?
I could never get my bike to idle consistently running the stock throttle cables (3 separate cables + splitter)
I ditched the standard set-up and went with Don Pender's carb gantry, super easy to synch carb slides & achieve a consistent even idle. (1 cable)